GaN-based HFET having a surface-leakage reducing cap layer

ABSTRACT

A semiconductor device includes: a substrate; a buffer layer including GaN formed on the substrate, wherein: surfaces of the buffer layer are c facets of Ga atoms; a channel layer including GaN or InGaN formed on the buffer layer, wherein: surfaces of the channel layer are c facets of Ga or In atoms; an electron donor layer including AlGaN formed on the channel layer, wherein: surfaces of the electron donor layer are c facets of Al or Ga atoms; a source electrode and a drain electrode formed on the electron donor layer; a cap layer including GaN or InGaAlN formed between the source electrode and the drain electrode, wherein: surfaces of the cap layer are c facets of Ga or In atoms and at least a portion of the cap layer is in contact with the electron donor layer; and a gate electrode formed at least a portion of which is in contact with the cap layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device, and moreparticularly to a field-effect transistor having a heterostructure of aGallium nitride-based semiconductor which is generally represented asIn_(X)Al_(Y)Ga_(1-X-Y)N (where 0≦X≦1, 0≦Y≦1).

2. Description of the Related Art

A Gallium nitride-based semiconductor such as GaN, AlGaN, InGan, InAlGaNor the like has high dielectric breakdown field, high thermalconductivity and a high electron saturation velocity, and thus ispromising as a material for a high-frequency power device. Particularlyin a semiconductor device having an AlGaN/GaN heterojunction structure,electrons accumulate at a high density in a heterojunction interfacebetween AlGaN and GaN, and a so-called two-dimensional electron gas isformed. This two-dimensional electron gas exists in a spatiallyseparated state from donor impurities added to AlGaN, and thus showshigh electron mobility. A field-effect transistor having such aheterostructure is produced so that source resistance can be reduced.Moreover, a distance d from a gate electrode to the two-dimensionalelectron gas is typically as short as tens of nm, and thus, even if agate length Lg is as short as about 100 nm, the ratio of the gate lengthLg to the distance d (i.e., aspect ratio) Lg/d, can be increased from 5to about 10. Accordingly, semiconductor devices having a heterostructurehave a superior feature in that a field-effect transistor which has aninsignificant short-channel effect and satisfactory saturation propertycan be readily produced. Moreover, a two-dimensional electron of theAlGaN/GaN-based heterostructure has an electron velocity in a high fieldregion of about 1×10⁵V/cm, which is twice or more than the speed ofAlGaAs/InGaAs-based heterostructure currently prevalent as ahigh-frequency transistor, and thus, is expected to be applied tohigh-frequency power devices.

A conventional semiconductor device 900 is shown in FIG. 9. Thesemiconductor device 900 is formed on a sapphire substrate or SiCsubstrate 901, on which the following layers are sequentially laminated;a buffer layer 902 including GaN; a channel layer 903 formed of GaN orInGaN; and an electron donor layer 904 including AlGaN. A sourceelectrode 906, a gate electrode 907 and a drain electrode 908 areprovided on the electron donor layer 904.

This AlGaN/GaN-based heterostructure is typically formed on a sapphiresubstrate or SiC substrate 901 composed of a (0001) facet (c facet),through a crystal growth process using a metal-organic chemical vapordeposition method or a molecular beam epitaxy method. In the case offorming the buffer layer 902 including GaN on the sapphire substrate orSiC substrate 901, it is necessary to thickly form the buffer layer 902in order to account for a great difference in lattice constant betweenthe substrate 901 and the buffer layer 902. This is because the straindue to a lattice mismatch between the buffer layer 902 and the substrate901 is sufficiently reduced by forming the buffer layer 902 so as tohave a relatively large thickness. By forming the electron donor layer904 containing AlGaN to which n-type impurities such as Si or the likeare added so as to have a thickness on the order of tens of nm on thisthick buffer layer 902, a two-dimensional electron gas (i.e., channellayer 903) is formed in the buffer layer 902 which has a great electronaffinity in the heterointerface between AlGaN and GaN due to the effectsof selective doping. The crystal facet of a heterostructure formed by anMOCVD (metal-organic chemical vapor deposition) method, is typicallycomposed of a facet of Ga, which is an III group element. Thintwo-dimensional electron gas is susceptible to the effects ofpiezo-polarization in a a axis direction due to tensile stress imposedon AlGaN, in addition to a difference in spontaneous polarizationbetween AlGaN (included in the electron donor layer 904) and GaN(included in the buffer layer 902). Thus, electrons accumulate at adensity which is higher than a value which would be expected from thedensity of the n-type impurities added to the electron donor layer 904.When Al composition of AlGaN of the electron donor layer 904 is 0.2 to0.3, electron density of the channel layer 903 is about 1×10¹³/cm²,which is about 3 times the density of a GaAs-based device. Since thetwo-dimensional electron gas of such a high density is accumulated, thesemiconductor device 900 used as a GaN-based heterostructurefield-effect transistor (FET) is considered as a highly promising powerdevice.

However, the conventional semiconductor device 900 has a number ofproblems as follows: (1) due to the imperfectness of crystal growthtechniques and their associated processes, a satisfactory crystal cannot be obtained; and (2) in the case of involving an etching process,the device properties may be deteriorated due to damage inflicted by theetching process, and thus, the expected power characteristics may not besufficiently realized.

One of the problems related to the crystal growth is associated with thefact that the undoped GaN included in the buffer layer 902 typicallyrepresents an n-type and the carrier density may be as high as about10¹⁶/cm³ or more. This is presumably because the constituent nitrogen(N) atoms are released during the crystal growth, and thus, vacanciesare liable to be formed. When there are such residual carriers, theleakage current component via the GaN buffer layer 902 of the devicebecomes greater. In particular, when operating the device at a hightemperature, deteriorations in the element properties such asaggravation of pinch-off characteristics may occur. As for an isolationproblem, when forming a plurality of GaN-based heterostructure FETs onthe same substrate, the FETs is interfere with each other to hindernormal operation. When the gate electrode 907 is further provided abovethis GaN buffer layer 902, a problem such as an increase of a gateleakage current, a drop in the voltage breakdown level of the device orthe like may arise.

As for problems associated with etching process technique, a facet ofGaN (included in the buffer layer 902) or AlGaN (included in theelectron donor layer 904) may be damaged. Since GaN and AlGaN aredifficult to remove or trim by means of wet etching, dry etching istypically performed for the etching process. However, a leakage currentis likely to flow in the surface of the buffer layer 902 or the electrondonor layer 904 due to the damage inflicted on the surface of the bufferlayer 902 or the electron donor layer 904. It is considered that inparticular, shortage of nitrogen on the surface increases theconductivity of the surface of the buffer layer 902 exposed by theetching, thereby increasing the leakage current.

SUMMARY OF THE INVENTION

In one aspect of the invention, a semiconductor device includes: asubstrate; a buffer layer including GaN formed on the substrate,wherein: surfaces of the buffer layer are c facets of Ga atoms; achannel layer including GaN or InGaN formed on the buffer layer,wherein: surfaces of the channel layer are c facets of Ga or In atoms;an electron donor layer including AlGaN formed on the channel layer,wherein: surfaces of the electron donor layer are a facets of Al or Gaatoms; a source electrode and a drain electrode formed an the electrondonor layer; a cap layer including GaN or InGaAlN formed between thesource electrode and the drain electrode, wherein: surfaces of the caplayer are c facets of Ga or In atoms and at least a portion of the caplayer is in contact with the electron donor layer; and a gate electrodeformed at least a portion of which is in contact with the cap layer.

In one embodiment of the invention, at least a portion of the gateelectrode may be formed so as to contact the electron donor layer.

In another embodiment of the invention, the gate electrode may be formedon the cap layer.

In still another embodiment of the invention, the cap layer may includeInGaAlN, the cap layer may have a composition which is substantiallylattice matched with the buffer layer in a c facet and the electrondonor layer may be formed so that an absolute value of a magnitude of apolarization occurring within the cap layer is smaller than that of amagnitude of a polarization occurring within the electron donor layer.

In still another embodiment of the invention, an n-type impurity may beadded to part or whole of the cap layer.

In still another embodiment of the invention, the gate electrode may bepositioned closer to the source electrode than to the drain electrode.

In still another embodiment of the invention, the gate electrode mayhave a surface area which is larger than that of the cap layer.

In still another embodiment of the invention, the gate electrode may bepositioned in a region where the cap layer is reduced in thickness orremoved.

In still another embodiment of the invention, the gate electrode may beformed an a side of the cap layer closer to the source electrode, andthe cap layer may be formed between the gate electrode and the drainelectrode.

In still another embodiment of the invention, the cap layer may includea semiconductor layer formed on the electron donor layer and aninsulating film formed on the semiconductor layer.

The present invention having the above-described structure enhances abarrier height of Schottky junction, thereby providing a semiconductordevice, which is capable of: reducing the leakage current as well aspreventing an increase of the source resistance; and/or improving thevoltage breakdown level as well as preventing an increase of the sourceresistance. Moreover, a region occupied by a cap layer between a gateelectrode and a drain electrode is made larger. Such a structure allowsthe voltage breakdown level of a semiconductor device to be improved.

In one aspect of the invention, a semiconductor device includes: asubstrate; a buffer layer including AlGaN formed on the substrate,wherein: surfaces of the buffer layer are c facets of N atoms; anelectron donor layer including AlGaN formed on the buffer layer,wherein: surfaces of the electron donor layer are a facets of N atoms; achannel layer including GaN or InGaN formed on the electron donor layer,wherein: surfaces of the channel layer are a facets of N atoms; a sourceelectrode and a drain electrode formed on the channel layer; a cap layerincluding AlGaN formed between the source electrode and the drainelectrode, wherein: surfaces of the cap layer are a facets of N atomsand at least a portion of the cap layer is in contact with the channellayer; and a gate electrode formed at least a portion of which is incontact with the cap layer.

In one embodiment of the invention, the gate electrode may be formed sothat at least a portion of which is in contact with the channel layer.

In another embodiment of the invention, the gate electrode may be formedon the cap layer.

In still another embodiment of the invention, the gate electrode may bepositioned, closer to the source electrode than to the drain electrode.

In still another embodiment of the invention, the gate electrode mayhave a surface area which is larger than that of the cap layer.

In still another embodiment of the invention, the gate electrode may bepositioned in a region where the cap layer is reduced in thickness orremoved.

In still another embodiment of the invention, the gate electrode may beformed on a side of the cap layer closer to the source electrode, andthe cap layer may be formed between the gate electrode and the drainelectrode.

In still another embodiment of the invention, the cap layer may includea semiconductor layer formed on the electron donor layer and aninsulating film formed on the semiconductor layer.

The present invention having the above-described structure enhances abarrier height of Schottky junction, thereby providing a semiconductordevice, which is capable of: reducing the leakage current as well aspreventing an increase of the source resistance; and/or improving thevoltage breakdown level as well as preventing an increase of the sourceresistance. Moreover, a region occupied by a cap layer between a gateelectrode and a drain electrode is made larger. Such a structure allowsthe voltage breakdown level of a semiconductor device to be improved.

Thus, the invention described herein makes possible the advantages of:(1) providing a semiconductor device (GaN-based heterostructure FET) inwhich a surface leakage current caused by residual carriers resultingfrom defects or damage accidentally caused in the interior or surface ofthe GaN layer is significantly reduced; and (2) providing asemiconductor device (GaN-based heterostructure FET) having a reducedsurface leakage current and improved voltage breakdown level.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view explaining a field-effect transistoraccording to Example 1 of the present invention.

FIG. 1B is a plan view illustrating a field-effect transistor accordingto Example 1 of the present invention.

FIG. 2 is a potential graph relative to Example 1 of the presentinvention.

FIG. 3 is a graph showing dependency of sheet electron density and peakpotential on a thickness of GaN cap layer relative to Example 1 of thepresent invention.

FIG. 4 is a cross-sectional view illustrating a field-effect transistoraccording to a variant of Example 1 of the present invention.

FIG. 5 is a cross-sectional view illustrating a field-effect transistoraccording to another variant of Example 1 of the present invention.

FIG. 6A is a cross-sectional view illustrating a field-effect transistoraccording to Example 2 of the present invention.

FIG. 6B is a cross-sectional view illustrating a field-effect transistoraccording to Example 2 of the present invention.

FIG. 6C is a cross-sectional view illustrating a field-effect transistoraccording to Example 2 of the present invention.

FIG. 6D is a cross-sectional view illustrating a field-effect transistoraccording to Example 2 of the present invention.

FIG. 6E is a cross-sectional view illustrating a field-effect transistoraccording to Example 2 of the present invention.

FIG. 7 is a cross-sectional view illustrating a field-effect transistoraccording to Example 3 of the present invention.

FIG. 8A in a cross-sectional view illustrating a field-effect transistoraccording to a variant of Example 3 of the present invention.

FIG. 8B is a cross-sectional view illustrating a field-effect transistoraccording to a variant of Example 3 of the present invention.

FIG. 8C is a cross-sectional view illustrating a field-effect transistoraccording to a variant of Example 3 of the present invention.

FIG. 8D is a cross-sectional view illustrating a field-effect transistoraccording to a variant of Example 3 of the present invention.

FIG. 8E is a cross-sectional view illustrating a field-effect transistoraccording to a variant of Example 3 of the present invention.

FIG. 9 is a cross-sectional view illustrating a conventionalfield-effect transistor.

DESCRIPTION OP THE PREFERRED EMBODIMENTS EXAMPLE 1

A semiconductor device according to Example 1 of the present inventionwill be described with reference to the figures. FIG. 1A is across-sectional view of a field-effect transistor (FET) 100 according toExample 1 of the present invention and FIG. 1B is a plan view thereof.The field-effect transistor 100 is formed on a substrate 101 composed ofsapphire or SiC, on which the following layers are sequentiallylaminated: a GaN buffer layer 102 having a thickness of about 2-3 μm; achannel layer 103 formed of GaN or InGaN; an n-type AlGaN electron donorlayer 104 having an AlN component ratio of about 0.15 to 0.5, to whichan n-type impurity such as Si is added at a density of about 2×10¹⁸cm⁻³; and a GaN cap layer 105 having a thickness of about 10-20 nm. TheGaN cap layer 105 is selectively etched so as to leave only a centralportion thereof. A gate electrode 107 is formed on the GaN cap layer105. A source electrode 106 and a drain electrode 108 are formed,adjacent to the gate electrode 107, on the surface of the AlGaN electrondonor layer 104 exposed after portions of the GaN cap layer 105 areremoved. The surfaces of each nitride layer are composed of c facets ofa III group element.

As shown in FIG. 1B, in the periphery of an element structural region109, a separation region 110 surrounding the element structural region109 is formed by means of a method which does not involve etching, e.g.,ion implantation. The GaN cap layer 105 is formed in an area which islarger than the gate electrode 107. Moreover, the GaN cap layer 105 isformed so as not to contact the source electrode 106 and the drainelectrode 108. The GaN cap layer 105 functions to enhance an effectivebarrier height (peak potential) of a Schottky electrode, as accountedfor by a difference between the magnitude of polarization occurring inthe GaN cap layer 105 and that occurring in the AlGaN electron donorlayer 104.

Next, the influence of polarization occurring when stress is imposed onthe field-effect transistor 100 having the above-described structurewill be described.

Since the GaN buffer layer 102 is sufficiently thick to reduce Acompression strain due to lattice mismatching, no piezo-polarizationoccurs due to the strain, but only spontaneous polarization occurs. Onthe other hand, the AlGaN electron donor layer 104 is subjected totensile strain, and substantial piezo-polarization occurs therein inaddition to the spontaneous polarization. This polarization occurs in ac axis direction of the substrate 101, i.e., a direction perpendicularto the upper surface of the substrate 101. FIG. 2 shows calculationresults of theoretical potential along a depth direction, using aninterface between the GaN cap layer 105 and the gate electrode 107 ofthe semiconductor device 100 shown in FIG. 1A as a point of reference(zero distance). The calculation takes into consideration theaforementioned influence from polarization.

In FIG. 2, the thickness of the GaN cap layer 105 as set to 10 nm and agate voltage is set to 0V. A potential difference occurs in the GaN caplayer 105 due to the influence of polarization. As a result, thepotential in a heterointerface with the AlGaN electron donor layer 104(the peak potential shown in FIG. 2) is increased, thereby increasingthe effective height of the Schottky barrier.

FIG. 3 shows calculation results of theoretical changes (marked by x inFIG. 3) in the effective barrier height (peak potential) where thethickness of the GaN cap layer 105 is varied from 0 to 20 nm, andchanges (denoted by “◯” symbols in FIG. 3) in the electron densityaccumulating at the heterointerface between the GaN cap layer 105 andthe AlGaN electron donor layer 104.

As shown in FIG. 3, while the effectual barrier height (peak potential)of the Schottky electrode gradually increases as the thickness of theGaN cap layer 105 increases, the electron density accumulating in theheterointerface between the GaN cap layer 105 and the AlGaN electrondonor layer 104 decreases. The reason why the peak potential increasesis that the barrier height of the Schottky electrode to the GaN caplayer 105 remains constant, whereas a potential difference occurring inthe GaN cap layer 105 increases along with an increase of a filmthickness of the GaN cap layer 105. Thus, the addition of the GaN caplayer 105 effectively increases the peak potential. The electron densitydecreases as the thickness of the GaN cap layer 105 increases because areverse bias is applied to the gate electrode by the potentialdifference residing in the GaN cap layer 105.

As described above, the provision of the GaN cap layer 105 increases thepeak potential and reduces the electron density accumulating at theheterointerface. All of these factors contribute to the high voltagebreakdown level of the resultant field-effect transistor. However, theleakage current includes a component which flows along the surfaces ofthe buffer layer 102. In particular, in the case of a material whichcreates a donor responsive to depletion of nitrogen atoms on thesurfaces, e.g., GaN included in the buffer layer 102, it is important toreduce the aforementioned component in the leakage current. Moreover, areduction in the electron density accumulating at the heterointerfaceleads to: an increase of resistance in a region where there is the GaNcap layer 105; an increase of source resistance of the field-effecttransistor; and a deterioration in performance of the transistor.

In the field-effect transistor 100 according to the present invention,the GaN cap layer 105 in the region between a gate and a source isremoved (i.e., the source electrode 106 and the cap layer 105 do notdirectly contact each other), thereby further reducing the sourceresistance. Moreover, the leakage current between the source and thegate, and the leakage current between the gate and the drain can bereduced because the GaN cap layer 105 is removed (i.e., the sourceelectrode 106 and the cap layer 105 do not directly contact each other,and the drain electrode 108 and the cap layer 105 also do not directlycontact each other). This is because, as has already been described, thepotential increases suddenly along a direction within the plane asindicated by an arrow a in FIG. 1B due to a potential differenceoccurring in the GaN cap layer 105, so that any electrons contributingto the leakage current will have to acquire a level of energy exceedingsuch increase in potential level. Electrons have an energy of about 26meV at room temperature. When the increase in potential level is 260meV, the leakage current can be decreased by about four orders ofmagnitude, which is an extremely significant reduction. In fact, as canbe seen from the variations in the peak potential in FIG. 3, theprovision of the GaN cap layer 105 with a 10 nm thickness, will allow anincrease in potential level of about 1 eV to be obtained as compared tothe case of not providing the GaN cap layer 105. As a result, a leakagecurrent value is expected to be further reduced.

FIG. 4 shows a field-effect transistor (FET) 400 which is a firstvariant of Example 1 of the present invention. The field-effecttransistor 400 differs from the field-effect translator 100 describedwith reference to FIG. 1A in that the field-effect transistor 400 isconstructed so that a portion of a GaN cap layer 405 on which a gateelectrode 407 is laminated is reduced in thickness or removed altogetherby etching. FIG. 4 shows an example of the gate electrode 407 whichcontacts a current donor layer 404. As described above, the GaN caplayer 405 is reduced in thickness or removed altogether and the gateelectrode 407 is laminated in a region where the GaN cap layer hasbecome thin or removed. Therefore, the deterioration of mutualconductance is prevented by the GaN cap layer 405. In this case,although the Schottky barrier height is not enhanced, the increase inpotential level along a direction parallel to the interface between theGaN cap layer and the AlGaN electron donor layer contributes to thereduction of the leakage current.

In the semiconductor device 100 shown FIG. 1A, an example of the surfacearea of the cap layer 105 which is greater than that of the gateelectrode 107 is shown; however, the present invention is not limited tothis. FIG. 5 shows a field-effect transistor (FET) 500 according to asecond variant of Example 1 of the present invention. The field-effecttransistor 500 is different from the field-effect transistor 100described with reference to FIG. 1A in that a GaN cap layer 505 has awidth which is smaller than that of a gate electrode 507. Accordingly,in the field-effect transistor 500, the gate electrode 507 is laminatedin a state extending beyond both sides of the GaN cap layer 505. Effectssuch as reduction in the leakage current and improvement in the voltagebreakdown level can be also attained by using this structure.

EXAMPLE 2

FIGS. 6A to 6E show cross-sectional views of field-effect transistors(FET) according to Example 2 of the present invention. Each of thefield-effect transistors shown in FIGS. 6A to 6E includes a GaN caplayer 605 in order to improve the voltage breakdown level.

A field-effect transistor (FET) 600 shown in FIG. 6A is different fromthe field-effect transistor (FET) 100 shown in FIG. 1 in that a gateelectrode 607 provided on the GaN cap layer 605 is disposed closer to asource electrode 606. Therefore, a depletion layer extending across achannel layer 603 immediately underlying the gate electrode 607 can belarger on a drain electrode 608 side and the voltage breakdown level ofthe field-effect transistor 640 can be improved.

A field-effect transistor 610 shown in FIG. 6B is different from thefield-effect transistor 600 shown in FIG. 6A in that the field-effecttransistor 610 has a structure in which a portion of the GaN cap layer605 on which the gate electrode 607 is formed is reduced in thickness orremoved altogether by etching. In the field-effect transistor 610 ofFIG. 6B, the GaN cap layer is etched so that the gate electrode 607contacts a current donor layer 604. In the field-effect transistor 610shown in FIG. 6B, the deterioration of mutual conductance can beprevented by introducing the GaN cap layer 605.

In a field-effect transistor 620 shown in FIG. 6C, the gate electrode607 is provided on the electron donor layer 604 and along a side edge ofthe GaN cap layer 605 closer to the source electrode 606. Accordingly,the GaN cap layer 605 is disposed between the gate electrode 607 and thedrain electrode 608. In accordance with the structure of thefield-effect transistor 620 shown in FIG. 6C, the leakage currentbetween the gate and the source is not improved, but the voltagebreakdown level between the gate and the drain is unproved. Especially,since the gate electrode 607 is formed along the side edge of the GaNcap layer 605 in a location closer to the source electrode 606, thefield concentration in the neighboring area of the gate electrode 607 onthe side that is facing the drain can be reduced and the voltagebreakdown level between the gate and the drain can be improved. In thesame manner as in the field-effect transistor 610 shown in FIG. 6B, anincrease of the source resistance can be prevented and the mutualconductance of FET can be improved.

The above-described example illustrates a case where GaN is used as thecap layer 605. However, a GaN cap layer 605 cannot be formed with arelatively large thickness. This is because, as shown in FIG. 3, anincreased thickness of GaN makes for insufficient sheet electron densityand/or an excessive peak potential so that positive holes may beaccumulated between the cap layer 605 and the electron donor layer 604.The need for providing a thick cap layer 605 without substantiallyaffecting the sheet electron density is especially pronounced in thefield-effect transistor 620 shown in FIG. 6C. This is because when thecap layer 605 of the field-effect transistor 620 has a large thickness,the field concentration in the neighboring area of the gate electrode607 on the side that is facing the drain is reduced, and the voltagebreakdown level of the field-effect transistor 620 is improved.Moreover, when the cap layer 605 of the field-effect transistor 620 hasa large thickness, a parasitic gate capacitance of the portion where thegate electrode 607 overlaps the cap layer 605 can be reduced, and highfrequency property of the field-effect transistor 620 can be improved.

There are two methods for providing the thick cap layer 605 maintainingproperly lowered sheet electron density as follows. A first method is touse an InGaAlN cap layer instead of using the GaN cap layer 605. Asecond method is to reduce a potential difference occurring in the caplayer by adding an n-type impurity to the cap layer.

The first method imposes a requirement on the composition of InGaAlN;that is, the lattice constant of the c facet must be substantiallymatched with that of the GaN buffer layer in order to provide a filmhaving a large thickness. For this purpose, since iN_(0.18)AL_(0.82)Nand GaN can be lattice matched, a mixed crystal of In_(0.18)Al_(0.82)Nwith GaN may be formed, resulting in (In_(0.18)Al_(0.82))_(x)Ga_(1-x)N.In practice, however some deviation in the composition may be allowed.Another requirement is that the magnitude of polarization inside theInGaAlN cap layer must be kept smaller than that of the polarizationoccurring in the AlGaN electron donor layer 604. This imposes aconstraint on the value of x in (In_(0.18)Al_(0.82))_(x)Ga_(1-x)N; themaximum value of x depends on the AlN component in the AlGaN electrondonor layer 604. The maximum value of x associated with typical AlNcomponent ratio in the AlGaN electron donor layer 604 can be calculatedas follows: when the AlN composition of the AlGaN electron donor layer604 is 10%, the maximum x value is about 0.16; and when the AlNcomposition of the AlGaN electron donor layer 604 is 30%, the maximum xvalue is about 0.47. The maximum x value may be considered to be about1.5 times the AlN component ratio in the AlGaN electron donor layer 604.

According to the second method, a proper thickness of the cap layer 605is determined by the density of the impurity added thereto. Although thematerial of the cap layer may be GaN or InGaAlN, the followingdescription of the second method conveniently assumes that GaN is used.The thickness of the cap layer can be increased under the followingconditions while keeping the potential similar to that in FIG. 2 in aregion underlying the AlGaN electron donor layer 104 (i.e., a regionspanning a distance of 10 nm or more in FIG. 2).

In FIG. 2, the surface potential of the cap layer 105 is fixed to theSchottky barrier height, i.e., 0.76V. An undoped GaN layer can be formedon the cap layer as thickly as possible by performing a doping so as toobtain an electric field which is substantially zero at the surface ofthe cap layer 105 and which is equal to the potential (about 1.6V) atthe interface between the cap layer 105 and the AlGaN electron donorlayer 104. Such requirements are calculated to give 16.7 nm as athickness of the cap layer and 3×10¹⁸/cm³ as a doping density of then-type impurity. An undoped GaN cap layer of a desirable thickness maybe formed on such an n-type GaN cap layer.

The above-described structure for the cap layer is merely presented asan exemplary embodiment of the invention, and the actual cap layer canbe designed with various combinations of density and thickness.Moreover, as in the field-effect transistors 610 and 620 shown in FIGS.6B and 6C, when the electrical charge control by means of the gateelectrode mainly occurs at a portion where the gate electrode 607 andthe field donor layer 604 contact with each other, the cap layer 605 maybe composed of a combination of a semiconductor layer 605 b (e.g., ann-type GaN layer) with an insulating film 605 a formed thereon, as Inthe field-effect transistors 630 and 640 shown in FIGS. 6D and 6E. ASiO₂ film or a silicon nitride film can be used as the insulating film.A silicon nitride film is more preferable because it is considered tohave a relatively low interface level density. The field-effecttransistor 630 shown in FIG. 6D includes the semiconductor layer 605 band the insulating film 605 a provided thereon instead of the cap layer605 of the field-effect transistor 610 shown in FIG. 6B; and thefield-effect transistor 640 shown in FIG. 6E includes the semiconductor605 b and the insulating film 605 a provided thereon instead of the caplayer 605 of the field-effect transistor 620 shown in FIG. 6C. In thefield-effect transistor 630, the gate electrode 607 is formed so as tocontact not only the AlGaN electron donor layer 604, but also the uppersurface of the cap layer 605. It will be appreciated that, in thefield-effect transistor 610 as well, the gate electrode 607 may beformed so as to contact not only the AlGaN electron donor layer 604 butalso the upper surface of the cap layer 605. Particularly, as describedabove, the voltage breakdown level can be expected to be improved byelongating the gate electrode 607 toward the drain electrode of the caplayer 605.

EXAMPLE 3

In each of the field-effect transistor (FET) structures described inExamples 1 and 2, the facet of the heterostructure is composed of a IIIgroup element. However, an alternative structure is required in the caseof forming the facet from a V group element. An example of such aheterostructure, the facet of which is composed of nitrogen, a V groupelement, will be described below.

FIG. 7 shows a field-effect transistor 700 as a specific example of theaforementioned structure. The field-effect transistor 700 is formed on asubstrate 701 composed of sapphire or SiC, on which the following layersare sequentially laminated: an AlGaN buffer layer 702 having a thicknessof about 2 to 3 μm and an AlN component ratio of about 0.15 to 0.5; ann-type AlGaN electron donor layer 703, to which an n-type impurity suchas Si is added at a density of about 2×10¹⁸ cm⁻³; a channel layer 704formed of GaN or InGaN having a thickness of about 15 to 20 nm; and anAlGaN cap layer 705 having a thickness of about 10 nm. In thisfield-effect transistor 700, the AlN component ratio of the respectiveAlGaN layers may be the same. However, when polarization effects aretaken into account, the AlN composition of the surface AlGaN cap layer705 can be prescribed to be greater than the AlN composition of theAlGaN buffer layer 702. As in the field-effect transistor 100 shown inFIG. 1A, the AlGaN cap layer 705 is selectively removed so as to leaveonly a central portion thereof. A gate electrode 707 is formed on theAlGaN cap layer 705. The source electrode 706 and the drain electrode708 are formed, adjacent to the gate electrode 707, on the channel layer704 after the AlGaN cap layer 705 are removed. As described above, thesurfaces of each nitride layer are composed of a facets of a V groupelement (nitrogen).

In the heterostructure field-effect transistor 700 mainly composed ofGaN, a set of growth conditions by a molecular beam epitaxy method forforming surfaces of a V group element, has already been reported. Whenproducing a film so that its surfaces will be formed of a V groupelement, the direction of polarization occurring in each layer isopposite to that in the case where the surfaces are composed of a IIIgroup element. Whereas the buffer layer 102 of the field-effecttransistor 100 shown in FIG. 1A is composed of GaN, the buffer layer 702of the field-effect transistor 700 is composed of AlGaN. An electrondonor layer 703 including AlGaN to which an n-type impurity such as Siis added and a channel layer 704 are sequentially formed on the bufferlayer 702. Electron supply to the channel layer 704 occurs via the AlGaNelectron donor layer 703 underlying the channel layer 704 as well as viaa positive electrical charge induced by the difference in polarizationbetween the channel layer 704 and the electron donor layer 703.Accordingly, the gate electrode is typically formed directly on thischannel layer 704. The AlGaN buffer layer 702 is formed so as to besufficiently thick so that the lattice strain is reduced. The channellayer 704 including GaN or InGaN is formed so as to be relatively thin,e.g., on the order of tens of nm, since the layer is subjected to acompression strain. As the cap layer 705, AlGaN is used instead of GaN.

Prevention of an increase of the source resistance and reduction of theleakage current are expected from such structure based on the samereason described in Example 1.

A number of variants are possible under Example 2; these variants areshown in FIGS. 8A to 8E in the form of field-effect transistors (FETs).However, in the field-effect transistors shown in FIGS. 8A to 8E, thesurfaces of each nitride layer are composed of a facets of a V groupelement (nitrogen).

A field-effect transistor 800 shown in FIG. 8A is constructed, as in thefield-effect transistor 400 shown in FIG. 4, so that a portion of anAlGaN cap layer 805 to form a gate electrode 807 is reduced in thicknessor removed by etching. Such a structure allows the introduction of theAlGaN cap layer 805 to prevent the deteriorating mutual conductance.

A field-effect transistor 810 shown in FIG. 8D corresponds to thefield-effect transistor 500 shown in FIG. 5. In the field-effecttransistor (PBT) 810, the gate electrode 807 is formed on the AlGaN caplayer 805, to and the AlGaN cap layer 805 has a smaller surface areathan that of the gate electrode 807. Accordingly, the AlGaN cap layer805 is formed so as to be within the bounds of the bottom surface of thegate electrode 807. A reduction of the leakage current and animprovement of the voltage breakdown level are expected by forming thefield-effect transistor 810 in the above-described way.

A field-effect transistor 820 shown in FIG. 8C corresponds to thefield-effect transistor 600 shown in FIG. 6A. The field-effecttransistor 820 has a gate electrode 807 provided on the AlGaN cap layer805, at a different position than in the field-effect transistor (FET)800 shown in FIG. 8A. The region occupied by the ALGaN cap layer 805between the gate electrode and the drain electrode is made larger bydisposing the gate electrode 807 so as to be closer to the sourceelectrode 806. Such a structure allows a depletion layer extendingacross a channel layer 804 directly underlying the gate electrode 807 tobe expanded toward a drain electrode 808 side, and the voltage breakdownlevel of the field-effect transistor 820 can be improved.

A field-effect transistor 830 shown in FIG. 8D corresponds to thefield-effect transistor 610 shown in FIG. 6B. The field-effecttransistor 830 is different from the field-effect transistor 820 shownin FIG. 8C in that a portion or the AlGaN cap layer 805 where the gateelectrode 807 is formed is reduced in thickness or removed. As in thestructure of the field-effect transistor 830, the deterioration inmutual conductance can be prevented by introducing the AlGaN cap layer805.

A field-effect transistor 840 shown in FIG. 8E corresponds to thefield-effect transistor 620 shown in FIG. 6C. The field-effecttransistor 840 has a structure in which the AlGaN cap layer 805 isprovided between the gate electrode 807 and the drain electrode 808.Although the structure of the field-effect transistor 840 may have nosignificant effect on the improvement in the leakage current between thegate electrode and the source electrode, it does provide for improvementof the voltage breakdown level between the gate electrode and the drainelectrode.

Prescribing a large thickness for the cap layer 805 is effective forimproving the voltage breakdown level between the gate electrode and thedrain electrode in the structure of the field-effect transistor 840.However, when surfaces are composed of a V group element, it is not easyto form a thick cap layer 805 by using any material other than AlGaN.The reason is that, unlike in the case where a facet of theheterostructure is formed of a III group element, due to the compressionstrain experienced by the GaN composing the channel layer 804 inside thefacet, the spontaneous polarization and the polarization caused bypiezoelectric effects occur in opposite directions, and as a whole, thepolarization occurring inside the GaN channel layer 804 has aconsiderably small absolute value. No material that results in a smallvalue of the polarization can be found among materials whose latticeconstants match that of the AlGaN buffer layer 802 except for AlGaN.Accordingly, it is more convenient and easier to dope the cap layer 805as described in Example 2 than to form a thick cap layer by usingmaterials other than AlGaN.

Moreover, the use of a combination of an AlGaN layer with an insulatingfilm formed thereon as the cap layer 805, as described in Example 2, isalso effective in the structure of the field-effect transistors 830 and840. An SiO₂ film or a silicon nitride film can be used as an insulatingfilm, but the use of a silicon nitride film is more preferable since anitride silicon film is considered to have a low interface leveldensity.

There have been previous reports on instances in which the GaN bufferlayers 102, 402, 502, or 602, and/or the AlGaN buffer layers 702, 802similar to those described in the present specification are formed onthe substrate 101, 401, 501, 601, 701, 801, respectively, with arelatively thin AlN layer (about 100 nm) interposed therebetween. Itwill be appreciated that the present invention is also applicable tosuch instance with no substantial changes.

The present invention provides a semiconductor device (field-effecttransistor), which is capable of: reducing the leakage current as wellas preventing an increase of the source resistance of the galliumnitride-based heterostructure; and/or improving the voltage breakdownlevel as well as preventing an increase of the source resistance. As aresult, it is possible to improve the power characteristics of thesemiconductor device of a gallium nitride-based heterostructure.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. A semiconductor device comprising: a substrate; abuffer layer comprising GaN formed on the substrate, wherein surfaces ofthe buffer layer are c facets of Ga atoms; a channel layer comprisingGaN or InGaN formed on the buffer layer, wherein surfaces of the channellayer are c facets of Ga or In atoms; an electron donor layer comprisingAlGaN formed on the channel layer, wherein surfaces of the electrondonor layer are c facets of Al or Ga atoms; a source electrode and adrain electrode formed on the electron donor layer; a cap layercomprising InGaAlN formed between the source electrode and the drainelectrode without being in contact with the source electrode and thedrain electrode, wherein surfaces of the cap layer are c facets of Ga orIn atoms and at least a portion of the cap layer is in contact with theelectron donor layer; and a gate electrode formed at least a portion ofwhich is in contact with the cap layer, wherein the cap layer has acomposition which is substantially lattice matched with the buffer layerin a c facet, and the electron donor layer is formed so that an absolutevalue of magnitude of a polarization occurring within the cap layer issmaller than that of a magnitude of a polarization occurring within theelectron donor layer.
 2. A semiconductor device according to claim 1,wherein at least a portion of the gate electrode is formed so as tocontact the electron donor layer.
 3. A semiconductor device according toclaim 1, wherein the gate electrode is formed on the cap layer.
 4. Asemiconductor device according to claim 1, wherein an n-type impurity isadded to part or whole of the cap layer.
 5. A semiconductor deviceaccording to claim 1, wherein the gate electrode is positioned closer tothe source electrode than to the drain electrode.
 6. A semiconductordevice according to claim 1, wherein the gate electrode is positioned ina region where the cap layer is etched so as to be reduced in thicknesscompared to the thickness of the cap layer as deposited or is etched soas to be removed.
 7. A semiconductor device according to claim 1,wherein: the gate electrode is formed on a side of the cap layer closerto the source electrode; and the cap layer is formed between the gateelectrode and the drain electrode.
 8. A semiconductor device accordingto claim 1, wherein the cap layer includes a semiconductor layer formedon the electron donor layer and an insulating film formed on thesemiconductor layer.